High field superconducting magnets are desired for improved resolution and signal to noise in high resolution nuclear magnetic resonance (NMR) spectroscopy. Such high field magnets typically use niobium-tin superconducting material. A niobium-tin wire is wound into a coil in which the component niobium and tin are in the unreacted state. The niobium-tin wire preferably has a rectangular cross section because it provides a higher filling factor for the winding of a superconducting magnet. The coil is heated to approximately 700.degree. C., thereby forming the superconducting intermetallic niobium-tin compound, Nb.sub.3 Sn.
These magnetic coils have two important characteristics: the magnetic field generated in the bore of the magnet has high homogeneity and is highly stable with time. High field stability is achieved by closing the circuit of the magnetic coil so that the current flows in a closed loop without resistance, i.e., is a superconductor. In order that the closed loop of the magnetic coil has no resistance, it is necessary that the joints closing the loop themselves have no resistance. Commonly used methods of preparing superconducting joints in metallic superconductors are inappropriate for use with intermetallic superconductors because the superconducting wire is thereby degraded and resistance is introduced into the loop.
John E. C. Williams et al. in "600 MHz Spectrometer Magnet" (IEEE Trans. Mag. 25(2), 1767 (1989)) have disclosed a superconducting joint as shown in FIG. 1 in which a round end of a niobium-tin wire 10 is diffusion bonded into the interior of a niobium-tin composite 12. The joint is completed by spot welding 13 of a niobium-titanium ribbon 14 to the outer surface of the niobium-tin composite. However, contact between the niobium-tin wire 10 and the niobium-tin composite 12 is not optimal because the area of contact between the wire and the composite is limited to only the perpendicular cross-section of the wire. Furthermore, wire with a rectangular cross-section can not be readily joined using this prior art superconducting joint.
It is the object of the present invention to provide a superconducting joint having superior electrical contact between pairs of superconducting wire in which at least one of the wires contains the superconductor niobium-tin.
It is a further object of the present invention to provide a hybrid niobium-tin/niobium-titanium superconducting joint that provides the processing flexibility of niobium titanium alloy with the desired magnetic properties of the niobium tin intermetallic compound.